Um, I don't think the concept's all that hard to grasp. Fertility rates as discussed in demographics are a statistical concept not a biological one. Given infant mortality you need approx 2.1 babies on average from every woman to reproduce the current population level. Fewer than that and the population begins to decline. More than that and it increases.
Experience shows that, in the modern world once the trend moves in one or other direction, it's immensely difficult to make it change back. The reason being that people in the modern world make conscious lifestyle choices under various social & economic influences.
People in the old worlds of poverty and ignorance did not make such choices as they were unaware such things as 'choice' existed. My MIL, 6 years older than me and illiterate, never been to school, has produced some - and I use the term advisedly - 11 children across 3 marriages. My FIL, 2 years older than me & similarly afflicted, has produced 8 children across 2 marriages.